Automatic Protection Switching (APS) refers to automated fault detection and corrective switching in a data communication network. When failure is detected in a particular transmission channel, communication resumes in a standby channel. Failure detection and recovery switching occur automatically, providing high reliability and availability through redundancy and automatic re-routing. APS in various forms has been developing for several decades such as with Synchronous Optical Network (SONET) and Synchronous Digital Hierarchy (SDH). Early variants of APS were applied in microwave systems and in some of the first asynchronous fiber networks. Currently, standardized forms of APS are required by many data communication network clients. Accordingly, network providers and planners are expected to comply with APS protocols and to provide some degree of reverse-compatibility with established APS standards even as network hardware and transmission media evolve.
According to recent APS convention, transmission was considered to occur from a head-end to a tail-end along a line designated as a “work line.” A separate “protect line” between the head end and tail end was designated for use if the work line fails. The head-end and tail-end were typically network elements that conventionally conducted bridging and selecting functions, respectively. A line could at one time be considered as a physical transmission route, such as along a particular electrical wire, along an optical fiber, or along a particular physical itinerary of such signal carriers. The work and protect lines in some cases were and are entirely physically redundant, with no significant hardware differences between the two. In many applications, there is no expectation or particular likelihood that the work line is more typically used than the protect line for active signal transmission. Thus, by some current terminology, an “active line” is tentatively designated as the signal carrying line, whether that be the work or the protect line.
In contrast to the technologies for which APS originated, a typical modern optical network element is bidirectional and, accordingly, need not be exclusively designated as a head-end or tail-end node. Modern networks have inherently redundant architectures considered as meshes, rings, or even clouds, in which discrete designations of work and protect lines are arguably obviated. Nonetheless, many network clients requiring APS compliance expect network transparency through two apparent channels, which are nominally termed as work and protect lines, either of which may serve as an active signal-carrying line as the other is held in reserve or utilized for low-priority “preemptible” traffic.
A protection switching request, directing traffic from a work line to a protect line, can be prompted automatically when loss or degradation of a signal is detected, for example at a tail-end. Protection switching requests can also be prompted by network carrier or client-side technicians and engineers. Switching-coordination bytes, APS “K-bytes” in SONET/SDH, for example, are typically placed in the overhead portions of client data-transmission frames to convey protection switching requests and confirmations. Despite whether discrete work and protect lines are only nominal or apparent in an inherently redundant network, many network clients expect APS K-bytes to be transparently conducted. However, corresponding overhead portions of client frames are typically terminated in favor or more native overhead formats as client data enters and transits a carrier network such as an Optical Transport Network (OTN) cloud. The contents of each of OTN references ITU-T G.709 (December 2009) “Interfaces for the Optical Transport Network (OTN)” and G.798 (October 2010) “Characteristics of optical transport network hierarchy equipment functional blocks” are incorporated by reference herein.
In 1+1 APS, a protect line is designated for each work line in one-to-one correspondence. In 1:N APS, multiple N work lines rely on a single shared protect line. While additional data such as low-priority traffic can be sent along a protect line at times when no faults are occurring on a work line, APS standards ultimately require some redundancy and therefore represent, at least to some degree, an inefficient use of network resources when conventionally applied. For example, only half of the actual network capacity for prioritized data is utilized when 1+1 APS protection is conventionally applied.
Conventionally, network deployments are moving toward OTN at the optical layer with SONET/SDH encapsulated therein. For example, a client data frame in SONET/SDH format is received and mapped into an Optical channel Data Unit (ODU) frame on an active APS line, the ODUk/j frame is transported over a carrier OTN network. At the other end of carrier network, the client SONET/SDH traffic is de-mapped from ODUk/j and handed off to client equipment. The hand-off can be a bridge when the client protocol is 1+1 APS. In such a case, both work and protect lines of the client side will receive identical SONET/SDH frames because the K-Bytes received from the client via the APS protect line are terminated on the send side of the OTN cloud if the APS work line is selected. That is, if the work line is selected at the input side of the OTN cloud, then the K-bytes received from the client via the work line at the input side are ultimately transmitted toward the client via both the work and protect lines at the output side of the OTN cloud. Specifically, the OTN network includes its own protection schemes such as mesh restoration and the like, and it is not efficient to transmit both the work line and the protect line of the 1+1 APS in the OTN network. In that example, K-bytes received from the client via the protect line at the input side are terminated without reaching the client at the output side. Similarly, if the protect line is selected, then K-bytes received from the client via the protect line at the input side are ultimately transmitted toward the client via both the work and protect lines at the output side, with the work line K-bytes at the input side being terminated. Hence, true bidirectional communication is not provided between the two APS groups on opposite sides of the OTN cloud.
By such exemplary mapping of SONET/SDH over OTN, the K1 and K2 bytes, for example as used in multiplex section protection (MSP), could be incorrectly transported if the client traffic is part of APS or other line protection protocols. This could cause client network elements to unnecessarily raise ambiguous alarms. Even worse, this could limit the support of client APS protocols such that only partial APS unidirectional switching is performed without raising any alarms according to APS, Bidirectional Line-Switched Ring (BLSR) and Transoceanic Line Switched Ring (TLSR) protocols. Because of such restrictions the client side protection cannot be BLSR, APS 1:N, APS 1:N+1, or TLSR, and truly bidirectional switching cannot be achieved. Furthermore, if junk K-bytes or inconsistent K-bytes are received at an input or an output side of an OTN cloud, there is no way of isolating and mitigating fault actions. If the work line is selected, K-bytes at the protect line are lost, and if the protect line is selected, K-bytes at the work line are lost.
Thus, improvements are needed for satisfying client expectations toward established APS protocols as handoffs from the OTN cloud while efficiently exploiting inherently redundant modern networks in the OTN cloud.